1. Field of the Invention
The present invention relates to a liquid-crystal display (LCD) device. More particularly, the invention relates to a LCD device comprising a first substrate and a second substrate coupled with each other, a liquid crystal layer formed in the gap between the first and second substrates with a sealing member, and spacers arranged in the gap, in which the uniformity of the gap is improved.
2. Description of the Related Art
As known well, the LCD device has a liquid crystal layer formed between two substrates and a pair of polarizer plates located outside of each of the substrates. Data voltages are applied across the liquid crystal layer to control the light penetrating through the said layer, thereby displaying images on its screen according to the data voltages applied.
FIG. 1 shows the structure of a prior-art LCD device, which is disclosed in the Japanese Non-Examined Patent Publication No. 9-73093 published in Mar. 18, 1997.
This prior-art device comprises a Thin-Film Transistor (TFT) substrate 112, an opposite substrate 114, and a liquid crystal layer 116 sandwiched by the substrates 112 and 114. A polarizer plate (not shown) is located on the outer surface of the substrate 112 and another polarizer plate (not shown) is located on the outer surface of the substrate 114. The polarizing axes of these two polarizer plates are perpendicular to each other. The combination of the two substrates 112 and 114, the liquid crystal layer 116, and the two polarizer plates constitute a LCD panel 126. The device is constituted by the panel 126 and other necessary parts such as driving circuitry (not shown).
When the device is seen from its front, it has a rectangular display region 118 for displaying images, and a frame-shaped non-display region 120 that surrounds the display region 118. In the display region 118, pixels and TFTs are arranged in a matrix array. The substrates 112 and 114 are coupled together with a sealing member 122 formed along the periphery of the non-display region 120. The liquid crystal layer 116 is made of a specific liquid crystal filled into the gap between the substrates 112 and 114. The layer 116 is sealed by the member 122. To keep the gap between the substrates 112 and 114 (i.e., the inter-substrate gap) uniform, columnar spacers 124 are formed in the liquid crystal layer 116. These spacers 124 are arranged at regular intervals within the display and non-display regions 118 and 120.
FIG. 2 shows the state in a prior-art method of fabricating the LCD device shown in FIG. 1, in which a TFT substrate member 112a and an opposite substrate member 114a are coupled to each other to simultaneously form the two LCD panes 126. Thus, this method includes the two-panel formation step. The two panels 126 are located in the middle of the coupled members 112a and 114a to be adjacent to each other. The liquid crystal layer 116 is not yet formed between the members 112a and 114a at this state.
As seen from FIG. 2, each panel 126, which is sectioned by the rectangular sealing member 122, includes the columnar spacers 124 regularly arranged in both the display region 118 and the non-display region 120. To ensure a desired gap between the substrate members 112a and 114a in their coupling step and to facilitate the subsequent cutting operation of the coupled members 112a and 114a, columnar auxiliary spacers 124a and auxiliary sealing members 128 are additionally formed. The auxiliary sealing members 128 are located outside the two panes 126, which are in the peripheral area of the coupled members 112a and 114a. The auxiliary spacers 124a are located outside the two sealing members 128.
The spacers 124 and 124a are fixed on the opposite substrate 114. As seen from FIG. 2, the density of the spacers 124 in the non-display region 120 of each panel 126 is higher than that of the spacers 124 in the display region 118 thereof. Therefore, when the substrate members 112a and 114a are coupled to each other and the sealing members 122 and 128 are cured, the spacers 124 located in the non-display region 120 of each panel 126 will fully withstand the relatively stronger pressing force applied to the region 120 than that applied to the display region 118 thereof. This means that the inter-substrate gap of each panel 126 in its peripheral part is less than that in its middle part.
The above-identified Publication No. 9-73093 further discloses another structure that the thickness or diameter of the columnar spacers 124 in the non-display region 120 of each panel is greater than that of the spacers 124 in the display region 118 thereof. In this structure, the same advantages as shown above are obtainable.
Unlike the above-described situation, the inventor found a fact that “inter-substrate gap non-uniformity” occurs in the neighborhood of the sealing member in each panel. The “inter-substrate gap non-uniformity” means that the inter-substrate gap of each panel in its peripheral part near the sealing member is greater than that in its remaining part. The cause of the gap non-uniformity the inventor found will be explained below with reference to FIGS. 3A to 3D and FIGS. 4A to 4C.
With the known liquid-crystal injection method, which is used in the ordinary fabrication method of the LCD device, the columnar spacers 124 have been formed on the inner surface of the opposite substrate member 114a while the sealing members 122 and 128 have been formed on the inner surface of the TFT substrate member 112a, as shown in FIG. 3A. Each of the sealing members 122 has a rectangular plan shape and each of the auxiliary sealing members 128 has a U- or L-like plan shape, as shown in FIG. 2. Spherical in-seal spacers 130 are arranged in each of the members 122 and 128, as shown in FIG. 3A. Needless to say, the columnar spacers 124 may be formed on the inner surface of the TFT substrate member 112a while the sealing members 122 and 128 may be formed on the opposite substrate member 114a. 
First, the substrate members 112a and 114a are coupled to each other with the sealing members 122 and 128 and then, these substrate members 112a and 114a are sandwiched by a pair of surface plates and applied with a pressing force by the plates. Alternately, the air existing in the gaps between the substrate members 112a and 114a is pumped out to reduce its inner pressure, thereby applying a pressing force to the members 112a and 114a thus coupled by the pressure difference in the atmospheric air. Due to the pressing force, the sealing members 122 and 128 and the opposite substrate member 114a are deformed and cured, thereby coupling the members 112a and 114a together and setting the gaps at their desired values. The state at this stage is shown in FIG. 3B.
In this coupling process, even if the applied pressing force is uniform over the whole substrate members 112a and 114a, the compressive deformation of the sealing members 122 and 128 is restricted due to the less deformation limits of the spacers 124. As a result, the gaps near the sealing members 122 and 128 are slightly larger than their desired value.
Subsequently, the coupled substrate members 112a and 114a are cut in such a way as to separate the two LCD panels 126. Then, a liquid crystal is injected into the gap of each panel 126 by way of its injection opening (not shown) penetrating the sealing member 122, thereby forming the liquid crystal layer 116 in the gap. In this injection process, the liquid crystal is injected until the opposite substrate 114 is slightly swelled out and the gap is somewhat larger than the height of the spacers 124, as shown in FIG. 3C.
To remove the extra liquid crystal from the gap, each panel 126 is sandwiched by a pair of surface plates and applied with a pressing force by the plates. Thus, the extra liquid crystal is pushed out from the gap by way of the injection opening. Finally, each panel 126 has the structure shown in FIG. 3D.
As seen from FIG. 3D, the inter-substrate gap between the TFT substrate 112 and the opposite substrate 114 is at its maximum value at the position of the sealing member 122 and at its desired value in the area away from the member 122. Since the inter-substrate gap decreases gradually from its maximum value to its desired value in the neighborhood of the member 122, a slope area S is formed in the opposite substrate 114. If the whole slope area S is located in the non-display region 120, no problem occurs. However, if part of the slope area S is located in the display region 118, as shown in FIG. 3D, a problem may occur. For example, if the slope angle of the area S in the display region 118 is equal to approximately 2% of the average value of the inter-substrate gap between two positions at an interval of 1 mm, a viewer recognizes the bad effect by the slope. This means that the image quality of the LCD device will degrade if the slope angle in the display region 118 satisfies the said relationship.
The image quality degradation will occur in the known liquid-crystal dropping and substrate coupling method as well, which is often used in the fabrication method of the LCD device. This is explained below with reference to FIGS. 4A to 4C.
Which one of the liquid-crystal injection method and the liquid-crystal dropping and substrate coupling method was used for fabricating a LCD device can be known by finding whether the injection opening exists in the sealing member or not. If the device has the injection opening in the sealing member, it is found that the device was fabricated by using the liquid-crystal injection method.
With the known liquid-crystal dropping and substrate coupling method, first, as shown in FIG. 4A, the columnar spacers 124 are formed on the inner surface of the opposite substrate member 114a while the sealing members 122 and 128 are formed on the inner surface of the TFT substrate member 112a. This is the same as in the liquid-crystal injection method shown in FIGS. 3A to 3D.
Thereafter, drops 132 of the liquid crystal are dropped onto the inner surface of the member 112a in each panel 126. Then, the opposite substrate member 114a is placed onto the TFT substrate member 112a in a vacuum atmosphere, thereby coupling the members 112a and 114a to each other with the sealing members 122 and 128, as shown in FIG. 4B. After the sealing members 122 and 128 are cured, the inside of the inter-substrate gap is kept in a vacuum state. Since the sealing member 122 needs to confine the drops 132 in the gap in this coupling process, the sealing member 122 has a higher viscosity than that of the member 122 used in the method of FIGS. 3A to 3D.
The substrate members 112a and 114a thus coupled are taken into the atmospheric air from the vacuum atmosphere. In this state, the members 112a and 114a are pressed by the atmospheric pressure. As a result, the inter-substrate gap is decreased and at the same time, the liquid-crystal drops 132 are expanded to form the liquid crystal layer 116, as shown in FIG. 4C. In this state, the peripheral area of each panel 126 is more difficult to be deformed than the middle area thereof. This is because the sealing member 122 is relatively higher in viscosity and because the spacers 124 near the member 122 provide resistance forces. As a result, the liquid crystal in the gap is likely to gather in the peripheral region of each panel 126. This means that the extra liquid crystal is likely to remain in the peripheral region near the member 122, keeping the gap at a larger value than the desired one near the sealing member 122.
As seen from FIG. 4C, the inter-substrate gap between the TFT substrate member 112a and the opposite substrate member 114a is at its maximum value at the position of the sealing member 122 and at its desired value in the area apart from the member 122. This is the same state as shown in FIG. 3D. Therefore, if part of the slope area S is located in the display region 118, as shown in FIG. 4C, the image quality of the LCD device will degrade.